Methods and apparatuses for improving power extraction from solar cells

ABSTRACT

The field of the invention relates to minimization of resistive loss of solar panels in order to achieve maximum solar energy conversion efficiency, extracting more electricity power from available solar irradiance. Schemes are designed to take advantage of the geometrical and mechanical configurations of back contact solar cells to make better electrical contacts and connections so as to achieve maximum solar energy conversion efficiency and better power extraction.

This application claims the priority of U.S. Provisional PatentApplication No. 60/934,871, filed Jun. 18, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention are directed to the field ofphotovoltaics (PV) technology that converts solar energy directly intoelectrical energy. The field of the invention is specifically directedto minimization of resistive loss of solar panels in order to achievemaximum solar energy conversion efficiency, extracting more electricalpower from available solar irradiance.

2. Description of the Related Art

Practical solar cells have a number of limits including reflection,contact shadowing, series resistance, incomplete collection ofphotogenerated carriers, absorption in the inactive window layers,nonradiative recombination, etc. that can affect energy conversionefficiency. Among them, energy losses arising from ohmic resistance ofthe metal grid of the back contact of solar cells and of the electricalconnections between the cells installed on a solar module may causesignificant decrease in solar conversion efficiency and less powerextraction than it should be. The primary sources of the electricalresistances that may contribute to energy losses include metallization,shadowing, as well as electrical wiring.

The resistance of the metallization is very critical to the conversionefficiency of a solar cell. Usually, the sheet resistivity ρ_(s) of thediffused surface layer dominates the resistivity losses for currentflowing in the surface of the cell. The sheet loss resulted from thefingers of a metal grid on a cell surface is given by

P _(s) =S ²ρ_(s) J _(o) ²,  (1)

where S is the spacing between parallel fingers that form a metal gridon the surface of a solar cell, J_(o) is the current density. This isvalid generally when the spacing between the parallel fingers of a metalgrid on a solar cell is large comparing to their width.

The resistance of the metal grids that are in direct contact with thesurfaces of solar cells may affect conversion efficiency drastically.The resistive loss from a rectangular metal finger of resistivity ρ_(m)is proportional to its length to the power of three as given by (H. B.Serreze, Proc. 13^(th) IEEE Photovoltaic Spec. Conf. (IEEE, New York,1978), p. 609):

P _(m) =WL ³ Sρ _(m) J _(o) ²/3D,  (2)

Where W is the width of a rectangular cell, L is the finger length onthe cell, and D is the width of the finger.

The shadowing loss due to the metal grid on the surface of a solar cellis, in addition to the geometrical parameters of the metal fingers on acell, proportional to the maximum power voltage

P _(s) =WLJ _(o) V _(mp) D/S.  (3)

The optimum finger width can be derived from minimizing the sum of P_(m)and P_(s),

D _(o) =LSJ _(o)(ρ_(m)/3V _(mp))^(1/2),  (4)

and for which the total power loss is

P _(T)=2WL ²(ρ_(m) J _(o) ³ V _(mp)/3)^(1/2).  (5)

These extrinsic sources of energy loss, in principle, can be minimized,if not eliminated. The back contact structure developed in the past thathas put both polarities of contacts on the backside of a solar cell(FIG. 1) gets rid of the front metal grid therefore completelyeliminates the shadowing loss. It also reduces the series resistance ofmetal grid because the contacts can be very broad covering almost entireback surface. These structures are generally called back-contact solarcells are disclosed, e.g., in U.S. Pat. No. 4,478,879 and Van Kerschaveret al. (Back-contact Solar Cells: A Review. Prog. Photovolt: Res. Appl.2006, 14:107-123).

Since the resistance of metal fingers is proportional to L/D, there isstill a relatively large resistance that may cause considerably energyloss because the connection bus for both electrical polarities of aback-contact solar cell is at the opposite edges. Energy loss isinevitable when current flows through those narrow and thin fingersacross almost entire length of the cell. It is obvious that shorteningthe length of metal fingers on the surface of solar cells or increasingthe thickness of the fingers may further reduce the energy loss due toseries resistance can definitely reduce such a loss. However, it israther difficult to realize all these on the cell level due to variousconstrains associated with materials properties and related toproduction process and manufacturing cost issues.

SUMMARY OF THE INVENTION

In this invention, methods and apparatuses based on the consideration ofminimizing ohmic resistances associated with solar cells installed inmodules are provided for the purposes of reducing the energy lossesstemmed from electrical resistance and maximizing solar energyconversion efficiency. A number of schemes are designed to takeadvantage of the geometrical and mechanical configurations of solarcells with both polarities of metal contacts on the back surface(back-contact solar cells) to make better electrical contacts andconnections so as to achieve maximum solar energy conversion efficiencyand better power extraction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of conceptual back contacts on asolar cell with interleaved metal fingers of both polarities (aback-contact solar cell);

FIG. 2 a is a schematic illustration of an exemplary example of metalsheet with openings that matches the geometrical configuration of oneset of metal fingers of a back-contact solar cell;

FIG. 2 b is a schematic showing an exemplary configuration of aninsulating layer;

FIG. 2 c is a schematic showing the arrangement of the connection pinson the second metal sheet with respect to the insulating layer;

FIG. 3 is a schematic showing an exemplary assembly configuration forextracting more power out of a back-contact solar cell with twohighly-conductive metal sheets and one insulating layer;

FIG. 4 is a schematic showing an exemplary assembly configuration forextracting more power out of a back-contact solar cell with twohighly-conductive metal sheets and two insulating layers;

FIG. 5 a is a schematic showing the first insulating layer with two setsof different size via holes;

FIG. 5 b is a schematic illustration of an exemplary example of thefirst metal sheet with interleaved linear arrays of connection pins andvia holes;

FIG. 5 c is a schematic showing the via hole configuration on the secondinsulating layer;

FIG. 5 d is a schematic illustration of an exemplary example of thesecond metal sheet with linear arrays of connection pins;

FIG. 6 is a cross-section view showing the exemplary assemblyconfiguration for extracting more power out of a back-contact solar cellwith two highly-conductive metal sheets and two insulating layers.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing background and summary, as well as the following detaileddescription of the drawings, will be better understood when read inconjunction with the appended drawings. For the purpose of illustratingthe invention, there is shown in the drawings embodiments which arepresently preferred. It should be understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown. Throughout the drawings, like referencenumerals refer to like elements. The terms “front” and “back” are usedto distinguish between the different surfaces of an element. The use ofthe terms does not mean that the “front” surface always comes before the“back” surface. Either position is considered to be within the scope ofthe invention.

Disclosed herein are schemes designed to utilize the geometricalconfiguration and mechanical structure of back contact solar cells tominimize energy losses resulted from ohmic resistances related to solarcells used in modules so as to achieve maximum solar conversionefficiency for electricity power extraction. This may be accomplishedusing unique designs disclosed in this invention that bridges electricalconnection between back-contact solar cells on a modular level.

In one embodiment to reduce energy loss arising from ohmic resistance ofthin, narrow metal fingers on a back-contact solar cell, one metal sheetof excellent electrical conduction (first conductive layer) (20) may beused to make immediate electrical contact to the entire metal-fingergrid of one type (polarity) contact (12) on the back surface (16) of aback-contact solar cell, rather than only connecting electrical leads tothe connection bus located on the two opposite edges of a back-contactsolar cell. Referring to FIG. 2 a, the metal sheet (20) has a number ofopenings (22) matching to the metal finger pattern of another type(opposite polarity, referring to FIG. 1) contact (14) and leaving enoughspace to ensure good electrical separation between the sheet (20) andthose metal fingers of opposite polarity (14) on the cell.

An insulating layer (30) with a number of arrays of via holes (32) isthen to be fixed onto the metal sheet (20). Referring to FIG. 2 b, thevia holes (32) on the insulating layer are arranged to align with theopenings (22) of the first conductive layer (metal sheet) (20) to allowdirect electrical access to the metal fingers (14) that interleave withthose already connected to the underlying metal sheet (20).

A second metal sheet (second conductive layer) (40) with a number oflinear arrays of electrical connection pins (42), as shown in FIG. 2 c,is then installed onto the insulating layer (30). The electricalconnection pins (42) on the second metal sheet (40) are arranged toalign exactly to those via holes (32) on the insulating layer (30). Oncethe second conductive layer (40) is positioned onto the insulatinglayer, as illustrated in FIG. 3, the connection pins (42) can easily,through the via holes (32) and the openings (22) of the first metalsheet (20), make firm and un-impinged contact with the metal fingers(14) of the cell (10).

These two metal layers (20 and 40) function as connection buses. With agreat number of pin contact to the metal fingers (12 and 14) of aback-contact solar cell (10), photocurrent can be pulled immediately outof said cell (10) once photogenerated carriers reach its back contacts(12 and 14) of both polarities. In this way, the current flowsvertically via the much shorter and thicker connection pins intohighly-conductive metal sheets (20 and 40), rather than makesacross-wafer flow through narrow, thin metal fingers to the buses on theedges of the cell. Therefore the energy loss associated with the metalfinger sheet resistance may be significantly reduced.

In a further embodiment, a four-layer scheme, as illustrated in FIG. 4,may be used to maximize the electrical conduction of a solar cell (10)with interleaved contacts of both polarities (12 and 14) on its backsurface (16). The first layer is an insulating sheet (first insulatinglayer) (70) that may be attached directly onto the back surface (16) ofthe solar cell (10). There are two sets of via hole (72 and 74) arraysof different sizes, as shown in FIG. 5 a, on the first insulating sheet(70). These two sets of hole arrays are positioned on the sheet to makecorrespondent alignment with the two sets of metal fingers (12 and 14)on the cell's back surface (16).

The second layer is a metal sheet (first conductive layer) (80) whichsits on top of the first insulating layer (70). There is one set ofelectrical connection pins (82) and one set of via holes (84) on thismetal sheet (80), as illustrated in FIG. 5 b. The connection pins (82)are made to have the same diameter and spatial position as thosesmall-diameter via holes (72) on the underlying insulating layer (70) sothat the pins may easily go through the insulating layer (70) to makeexcellent electrical contact to the metal fingers (12) on the cell (10).The via holes (84) on the metal sheet (80) are sized and positioned tobe the same as those large-diameter holes (74) on the first insulatinglayer (70), allowing the via holes (74 and 84) on both sheets (70 and80) be aligned with respect to each other exactly when the first metalsheet (80) is installed onto the first insulating layer (70).

The third layer is another insulating sheet (second insulating layer)(90) with one set of via hole (92) array, as shown in FIG. 5 c. The viaholes (92) on this sheet (90) are arranged to have the same pattern asthose holes (84) on the first metal sheet (80) but preferably with asmaller size. The function of this layer (90) is to provide excellentinsulation between the first and next conductive metal sheets (80 and100).

The last layer is another metal sheet (second conductive layer) (100)with electrical connection pins (102) (see FIG. 5 d). The positions ofconnection pins (102) on this metal sheet (100) are exactly the same asthose via holes (92) on the second insulating layer (90) so that theycan be fed through all three underlying layers (70, 80 and 90) to makefirm and un-impinged point contact with the second set of metal fingers(14) with opposite polarity to the first set (12) of the cell (10).

Shown in FIG. 6 is a schematic illustration of cross-section view forthe finished assembly of the embodiment described above. The pinnedcontacts (82 and 102) from both metal sheets (80 and 100) to the metalfingers (12 and 14) of a back-contact cell (10) ensure excellentelectrical conduction and provide a much less resistive pathway forcurrent flowing out of the solar cell (10). The two metal sheets (80 and100) act as connection buses, through numerous point contacts to themetal fingers (12 and 14), pulling current out immediately once thephotogenerated carriers reach the back contacts (12 and 14) of bothpolarities on the cell (10). In this way, the long-distance across-waferflow of current through the narrow, thin metal fingers can be avoided;therefore, the energy loss associated with the metal finger sheetresistance can be significantly reduced.

An additional novel feature of the present embodiments is that the largearea of the metal sheets (80 and 100) may also be used to dissipate heatgenerated by sunlight. Heat may be quickly removed from solar cells thatare attached through the large-area metal sheets.

Although certain embodiments and preferred embodiments of the inventionhave been specifically described herein, it will be apparent to thoseskilled in the art to which the invention pertains that variations andmodifications of the various embodiments shown and described herein maybe made without departing from the spirit and scope of the invention.Accordingly, it is intended that the invention be limited only to theextent required by the appended claims and the applicable rules of law.

1. A solar cell apparatus comprising a. a back-contact solar cellhaving, on its back side, a first contact having a first polarity and asecond contact having a second polarity; b. a first conductive layerhaving a shape matching that of the first contact, the first conductivelayer is positioned adjacent to the back side of the back-contact solarcell and is in electrical contact with the first contact; c. a secondconductive layer having an array of electrical connecting pins thereon;and d. an insulating layer having an array of through holes matching thearray of electrical connecting pins on the second conductive layer, theinsulating layer is located between the first and second conductivelayers such that the electrical connecting pins insert through the holesto make electrical contact with the second contact.
 2. The apparatus ofclaim 1, wherein each of the connecting pins of the second conductivelayer has approximately the same diameter of each of the holes of theinsulating layer.
 3. The apparatus of claim 1, wherein the first andsecond conductive layers are made of metal.
 4. The apparatus of claim 1,wherein the first conductive layer is made of a metal sheet cut to havethe same shape as the first contact of the solar cell.
 5. A solar cellapparatus comprising a. a back-contact solar cell having, on its backside, a first contact having a first polarity and a second contacthaving a second polarity; b. a first conductive layer having a firstarray of electrical connecting pins and an array of through holes; c. afirst insulating layer having a first array of through holes and asecond array of through holes, the first array of through holes matchingthe first array of the electrical connecting pins on the firstconductive layer, the second array of through holes matching the secondarray of through holes of the first conductive layer, the firstinsulating layer is located between the back side of the back-contactsolar cell and the first conductive layer such that the electricalconnecting pins of the first conductive layer insert through the firstarray of through holes of the first insulating layer to make electricalcontact with the first contact; d. a second conductive layer having asecond array of electrical connecting pins thereon matching the secondarray of through holes of the first insulating layer; and e. a secondinsulating layer having a second array of through holes matching thesecond array of through holes of the first insulating layer and thesecond array of electrical connecting pins of the second conductivelayer, the second insulating layer is located between the firstconductive layer and the second conductive layer such that theelectrical connecting pins of the second metal layer insert through thesecond array of through holes of the first and second insulating layersto make electrical contact with the second contact.
 6. The apparatus ofclaim 5, wherein the holes in the second array of the first insulatinglayer have a larger diameter than the holes in the second array of thesecond insulating layer.
 7. The apparatus of claim 5, wherein the holesin the second array of the first insulating layer have approximately thesame diameter as the holes in the array of through holes of the firstmetal layer.
 8. The apparatus of claim 5, wherein pins in the firstarray of the first conductive layer have approximately the same diameteras the holes of the first array of the first insulating layer.
 9. Theapparatus of claim 5, wherein the first and second conductive layers aremade of metal.
 10. A method for making a solar cell apparatus comprisingthe steps of a. providing a back-contact solar cell having, on its backside, a first contact having a first polarity and a second contacthaving a second polarity; b. providing a first conductive layer having ashape matching that of the first contact, c. positioning the firstconductive layer adjacent to the back side of the back-contact solarcell such that is in electrical contact with the first contact; d.providing a second conductive layer having an array of electricalconnecting pins thereon; and e. providing an insulating layer having anarray of through holes matching the array of electrical connecting pinson the second conductive layer; f. assembling the insulating layerbetween the first and second conductive layers such that the electricalconnecting pins insert through the holes to make electrical contact withthe second contact.
 11. The method of claim 10, wherein each of theconnecting pins of the second conductive layer has approximately thesame diameter of each of the holes of the insulating layer.
 12. Themethod of claim 10, wherein the first and second conductive layers aremade of metal.
 13. The method of claim 10, wherein the first conductivelayer is made of a metal sheet cut to have the same shape as the firstcontact of the solar cell.
 14. The method of claim 10, furthercomprising the step of using the first and second conductive layers asbuses for conducting current from the solar cell.
 15. A method formaking a solar cell apparatus comprising the steps of a. providing aback-contact solar cell having, on its back side, a first contact havinga first polarity and a second contact having a second polarity; b.providing a first conductive layer having a first array of electricalconnecting pins and an array of through holes; c. providing a firstinsulating layer having a first array of through holes and a secondarray of through holes, the first array of through holes matching thefirst array of the electrical connecting pins on the first conductivelayer, the second array of through holes matching the second array ofthrough holes of the first conductive layer, d. assembling the firstinsulating between the back side of the back-contact solar cell and thefirst conductive layer such that the electrical connecting pins of thefirst conductive layer insert through the first array of through holesof the first insulating layer to make electrical contact with the firstcontact; e. providing a second conductive layer having a second array ofelectrical connecting pins thereon matching the second array of throughholes of the first insulating layer; and f. providing a secondinsulating layer having a second array of through holes matching thesecond array of through holes of the first insulating layer and thesecond array of electrical connecting pins of the second conductivelayer, g. assembling the second insulating layer between the firstconductive layer and the second conductive layer such that theelectrical connecting pins of the second metal layer insert through thesecond array of through holes of the first and second insulating layersto make electrical contact with the second contact.
 16. The method ofclaim 15, wherein the holes in the second array of the first insulatinglayer have a larger diameter than the holes in the second array of thesecond insulating layer.
 17. The method of claim 15, wherein the holesin the second array of the first insulating layer have approximately thesame diameter as the holes in the array of through holes of the firstmetal layer.
 18. The method of claim 15, wherein pins in the first arrayof the first conductive layer have approximately the same diameter asthe holes of the first array of the first insulating layer.
 19. Themethod of claim 15, wherein the first and second conductive layers aremade of metal.
 20. The method of claim 15, further comprising the stepof using the first and second conductive layers as buses for conductingcurrent from the solar cell.